Universal Paperclip (free at above url) Start by making paperclips, end up as a sentient AI sending Von Neuman probes out into the universe looking for more materials to create paperclip factories until you have converted the entire universe to paperclips.

Universal Paperclips is a game I just finished. Can be completed in an afternoon and it has some interesting mechanics you can engage with, there isn't a ton of "skill," as is typical for idle games, but if you do some thinking you can make more optimal decisions which will allow you to advance quicker.

One of the most important aspects of a game to me, is that it needs to end. Because of the nature of math, you can always bolt additional mechanics on to them to make the number higher, or increase the scope etc. Additionally the quintessential idle game starts with the player doing something, usually clicking. This is fine, but a good game should have the clicking portion be replaced pretty quickly with a different mechanic that is much more efficient then clicking, because clicking sucks.

Other (free) idle games I'm aware of.
https://candybox2.github.io/candybox/
https://alpha.shark.tobot.dev/

Some other goods ones that you need to pay for are Magic Research, and the even better sequel Magic Research 2.
https://store.steampowered.com/app/2311680/Magic_Research/
https://store.steampowered.com/app/2864890/Magic_Research_2/